Field of the Invention
It is well known that in perforating earthen formations to tap subterranean deposits such as gas or oil, that perforation is accomplished by well drilling tools and a drilling fluid. These rotary drilling systems consist of a drilling bit fitted with appropriate `teeth`, then a set of pipes assembled rigidly together end to end, the diameter of which is smaller than that of the drilling bit. This whole rigid piece of equipment, drill bit and drill pipe string, is driven into rotation from a platform situated above the well being drilled. As the drill bit attacks and goes through the geological strata, the crushed mineral materials must be cleared away from the bottom of the hole to enable the drilling operation to continue. Aqueous clay dispersion drilling fluids are recirculated down through the hollow pipe, across the face of the drill bit, and upward through the hole.
The drilling fluid serves to cool and lubricate the drill bit, to raise the drilling cuttings to the surface of the ground, and to seal the sides of the well to prevent loss of water and drilling fluids into the formation through which the drill hole is being bored. After each passage through the well, the mud is passed through a settling tank or trough wherein the sand and drill cuttings are separated, with or without screening. The fluid is then again pumped into the drill pipe by a mud pump.
Some of the most serious problems encountered in producing and maintaining effective clay-based aqueous drilling fluids are caused by the interaction of the mud with the earth formation being drilled. These interactions include contamination of the mud by formation fluids, incorporation into the mud of viscosity producing and inert drilled solids, chemical contamination by drilled solids, or by the infiltration of sea-water and/or fresh water. The conditions of high temperature and pressure inherent with deeper and deeper drilling operations, together with formation interactions, make drilling fluid behavior unreliable and difficult to reproduce.
Characteristics of a ideal drilling fluid would then include the following:
i) To have rheological characteristics as desirable as possible to be able to transport the mineral cuttings set in dispersion. PA1 ii) To allow the separation of cuttings by all known means as soon as the mud flows out of the hole. PA1 iii) To have such required density as to exert sufficient pressure on the drilled geological formations. PA1 iv) To retain its fundamental rheological qualities as it is submitted, in very deep drilling, to higher and higher temperatures.
Ammonium and alkali metal salts of polymers of acrylic acid have been taught for use as a protective colloid for water based drilling fluids, as for example in U.S. Pat. Nos. 2,552,775 and 3,764,530. Further, U.S. Pat. Nos. 3,706,717 and 3,203,938 teach the use of copolymers of acrylic acid, acrylamide, or mixtures thereof and alkali metal ethylene sulfonates in drilling muds. Polyacrylic acid alone and with sulfonated styrene-maleic anhydride copolymer were taught as high temperature dispersants for water based bentonite clay drilling fluids in U.S. Pat. No. 4,476,029. U.S. Pat. No. 4,680,128 teaches the use of acrylic acid-vinyl sulfonic acid copolymers as dispersants and high temperature deflocculants for the stabilization of the rheological properties of aqueous clay based drilling fluids. U.K. Patent Application GB 2,133,021 A teaches the use of polymers made from acrylic acid, acrylamide, and a phosphate ester of ethylene glycol monomethacrylate as a thinning adjuvant for sea water based drilling muds. U.S. Pat. Nos. 4,728,445 and 4,721,576 teach the use of sulfonated styreneitaconic acid copolymers as improving the rheological characteristics of aqueous well drilling fluids. U.S. Pat. No. 4,770,795 teaches the use of copolymers of sulfoethyl and/or sulfophenyl acrylamide as calcium tolerant deflocculants for drilling fluids.
The polymers of the current art do not provide adequate predictable drilling fluid rheology after exposure to the common downhole hazards. Moreover, these drilling fluids tend to flocculate after exposure to typical deep well drilling conditions.
It is an object of the present invention to provide a method of preventing the flocculation of drilling muds and maintenance of adequate drilling fluid rheology after exposure to typical deep well drilling conditions.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.